The present invention generally relates to a braze-based protective coating for a Si-based substrate. The present invention also relates to components having a braze-based protective coating, and to methods of making such components.
Hot section components, such as blades, bladed disks (blisks), nozzles, turbine shrouds, and combustors, made from silicon-based materials, such as silicon nitride (Si3N4), silicon carbide (SiC), and their composites, have the potential to increase the operating temperatures of gas turbine engines, as compared with Ni-based superalloys. However, in the gas turbine environment, such silicon-based materials are prone to excessive oxidation to form a silica layer, and the subsequent rapid loss of the silica layer. The silica layer reacts with water vapor in the combustion gas environment to form a volatile Si(OH)4 species. The combination of excessive oxidation of silicon-based components and continual thinning of the resulting silica layer by Si(OH)4 evaporation leads to recession of the components, reduced load-bearing capability, and shortened lifetime.
Attempts have been made to inhibit oxidation of silicon-based components by applying an environmental barrier coating to prevent direct exposure of the silica layer or substrate to oxygen and water vapor in the gas turbine engine environment. However, reaction of the silica layer with constituents of the environmental barrier coating (EBC) at the substrate-EBC interface degrades the protective property of the silica layer. Similarly, the silica layer may be degraded by reaction with constituents of the substrate with which it may be in contact. Further, silica layers of the prior art which are in direct contact with Si-based substrates continue to grow in thickness until through-thickness cracks develop, this may lead to spallation of an entire environmental barrier coating.
Prior art processes for coating silicon-based substrates, such as plasma spray and electron beam-physical vapor deposition (EB-PVD), are expensive. Further, such line-of-sight coating processes are not suited to depositing a uniform coating on substrates of complex geometry, such as multi-airfoil components (e.g., integral nozzles and integral turbine wheels, or blades).
Thus, there is a need for a high temperature (>2200° F.) oxidation barrier for Si-based gas turbine engine components. There is an additional need for a protective coating for a Si-based substrate, wherein the protective coating includes an oxidation barrier disposed on the Si-based substrate, and an environmental barrier coating disposed on the oxidation barrier. There is also a need for a low cost process for forming a dense, uniform layer of the oxidation barrier on a Si-based component of complex geometry, such as an integral nozzle or an integral turbine bladed disk (blisk).